The following publications are believed to represent the current state of the art:
U.S. Pat. Nos. 5,805,245; 6,009,400; 6,685,094; 6,758,397; 7,562,811; 8,091,776 and RE 39,266; and
U.S. Published Patent Application Nos.: 2009/0230182, 2010/0219235, 2011/0006109 and 2011/0006115.